A dye-sensitized solar cell has excellent advantages that (1) the production cost thereof is expected to be ⅕ to 1/10 of that of a silicon-based solar cell, and thus is inexpensive, (2) the amount of emission of CO2 during the production thereof is low as 1/10 or less of that of a single crystalline silicon solar cell, (3) the energy payback time or CO2 payback time is short as half or less of that of a polycrystalline silicon solar cell, (4) raw materials are limited little in view of resources, (5) it is excellent in design and processability, and thus the surface area is readily increased, (6) the dye-sensitized solar cell has a relatively high photoelectric conversion efficiency of 10% or more that is comparable to that of an amorphous silicon solar cell, and the like, and therefore developments and studies are actively proceeded as a next-generation solar cell that takes the place of a silicon-based solar cell.
As shown in FIG. 1, a dye-sensitized solar cell 10 is constituted by a photoelectrode (photoelectric conversion element) 11 and a counter electrode 12, and an electrolytic composition (electrolyte) part 13 that is interposed between the two electrodes. The photoelectrode 11 is prepared by applying nanosize TiO2 particles 15a to the electroconductive film side of a transparent electroconductive substrate 14 in which an electroconductive film 14b is formed on the surface of a substrate 14a such as glass and calcinating the particles to form a porous TiO2 thin film 15b, and fixing a sensitizing dye 15c on the porous TiO2 thin film 15b by chemical adsorption. The counter electrode 12 is prepared by subjecting the electroconductive film side of a transparent electroconductive substrate 16 in which an electroconductive film 16b is formed on the surface of a substrate 16a such as glass to a treatment with a catalytic amount of platinum 17 or a treatment with electroconductive carbon. A solar cell is prepared by superposing the photoelectrode 11 and counter electrode 12, and injecting the electrolytic composition 13 containing an iodine compound into the gap between the electrodes 11 and 12.
Furthermore, as shown in FIG. 2, the power generation mechanism of a dye-sensitized solar cell is that electrons are injected from a sensitizing dye that has been excited by irradiation of solar light (visible light) to a conduction band of TiO2, the injected electrons are brought to an external circuit through a photoelectrode and transferred to a counter electrode, and the sensitizing dye (dye cation) in an oxidized state receives and recovers the electrons through a redox reaction of the electrolytic composition. By this cycle, photoelectric conversion is achieved.
However, a dye-sensitized solar cell has not been put into practical use yet since it has a lower photoelectric conversion efficiency than those of silicon-based solar cells that are commercially available at present. The main cause of the decrease of the photoelectric conversion efficiency of a dye-sensitized solar cell resides in the decrease of a voltage due to reverse electron transfer from a TiO2 layer to an electrolytic composition and dye cation (see FIG. 3), and in order to suppress reverse electron transfer to prevent voltage descending, addition of a basic heterocyclic compound to an electrolytic composition is studied. R1 and R2 in FIG. 3 represent reverse electron transfer (charge recombination).
Currently, 4-tert-butylpyridine (4-TBpy) is known as an additive for an electrolytic composition which is the most effective in the improvement of the conversion efficiencies of dye-sensitized solar cells. In the case when 4-TBpy is added as an additive for an electrolytic composition to an electrolytic composition, the increase of an open circuit voltage and the improvement of a fill factor occur, and a photoelectric conversion efficiency, that is calculated by multiplying a short-circuit current, an open circuit voltage and a fill factor, rises.
However, since an open circuit voltage increases but a short-circuit current decreases in 4-TBpy, a photoelectric conversion efficiency could not be significantly improved. Therefore, an additive for an electrolytic composition which can suppress the decrease of a short-circuit current and improve an open circuit voltage and a fill factor has been required.
As the above-mentioned additive for an electrolytic composition that takes the place of 4-TBpy, use of an additive for an electrolytic composition containing a compound having at least one or more of isocyanate group is disclosed (for example, see Patent Literature 1). According to the above-mentioned Patent Literature 1, by using the compound having at least one or more of isocyanate group, an open circuit voltage can be improved without causing significant decrease in a short-circuit current in a dye-sensitized photoelectric conversion element, thereby a photoelectric conversion efficiency can be improved.